Many materials are known to not absorb microwave energy or other electromagnetic radiation. Such materials may be reflective or transparent to the electromagnetic radiation without being affected thereby. Therefore, these materials do not heat when exposed to microwave or radiowave fields.
Many materials are known to absorb electromagnetic radiation and therefore will heat. Heating with microwave energy is but one example of this phenomenon, and many compositions that heat upon absorption of microwave energy are known. For example, water, fats, and certain food products absorb microwave energy and are heated thereby. Similarly, inorganic compounds such as carborundum powder, ferrites, zinc oxide, silicon carbide, and even carbon particles are known to heat upon absorption of microwave energy. Such compounds can be used to impart heat to their surroundings.
However, not every form of such materials can be used in this way. For example, metal powder can be used to absorb microwave energy, and is used in combination with other compositions to form heatable objects. However, metal is highly conductive, and this high conductivity can lead to arcing or sparking. For example, a large mass of solid metal typically cannot be placed in a microwave oven without causing damage from arcing and sparks caused thereby.
More than 200 crystal structures and forms of silicon carbide have been identified. Some forms of silicon carbide are known to heat upon absorption of microwave energy, and can be used in various forms in manufacture of objects that heat by absorption of electromagnetic radiation. Silicon carbide thus used is polycrystalline particulate. Silicon carbide is used to make objects that are heated by absorption of microwave energy. Such objects are used, for example, as reversible water absorbents, i.e., objects that absorb water that then are dried and rejuvenated by heating in a microwave oven to drive off the absorbed water. Other forms of silicon carbide are known to be minimally absorptive of microwave radiation.
Absorptive forms of silicon carbide can be used as a part of products used for heating foods. In particular, silicon carbide is used for manufacture of heating objects used to form a browned surface on foods heated in a microwave oven, because microwave energy alone often does not brown foods. Also, silicon carbide whiskers are described as mineral filler, providing rigidity and strength, for resins used to make containers used in microwave cooking. When used as a mineral filler, silicon carbide usually is in a form that is minimally absorptive of microwave radiation.
Silicon carbide also is used in selected steps of processes for manufacture of ceramic and metallic objects. For example, organic binder has been removed from an object made from silicon carbide powder by heating in microwave energy. Also, silicon carbide powder is known as a microwave-absorbent material suitable for heating ceramic pellets buried therein to degrease the pellets, or to remove binder and sinter the pellets. Graphite, silicon carbide, and other di-electric materials are known to be suitable material to be embedded in a polymeric ceramic precursor system containing a metal element that is, upon exposure to high-frequency (greater than 20 GHz) microwave energy, cured by heat produced by the di-electric to form a ceramic/metal composite. However, these systems typically have slow heating rates, thereby negating the benefits associated with microwave heating.
It is known that exposing silicon carbide, ceramic fibers, and microwave absorptive materials to microwave energy may lead to undesirable arcing and sparking. Metals, including metal powders, also lead to arcing because metals are conductive. Therefore, methods of reducing such sparking have been developed. In one such method, silicon carbide is deposited in and around the ceramic fibers by chemical vapor deposition. Such ceramic/silicon carbide composites heat when exposed to microwave energy, but the silicon carbide formed by CVD suppresses sparking.
Silicon carbide and other carbon-containing materials are mixed into a ceramic-containing powder mixture to serve as an aid to the heating of a ceramic powder thick-walled object during sintering. The powder contains materials, such as clays and kaolins, to increase susceptibility to microwave energy, and the object is autoclaved before microwaving to put the ceramic powders into a form that absorbs microwave energy more efficiently than the unchanged ceramic. The object then is sintered by exposure to microwave energy. However, if the object is not autoclaved, the heating rate is unacceptably slow.
There exist limitations on the use of silicon carbide in heat-generating objects. The heating efficiency of even the most absorptive forms of silicon carbide typically is low. Indeed, the heating efficiency of typical silicon carbide particulate is so low that such particulate is used to increase the strength and cut-resistance of microwave-heatable food containers that remain cool to the touch. It also has been disclosed that addition of silicon carbide to plastics of various types yields a product that imparts objectionable odors during use. Thus, use of such silicon carbide forms requires ameliorating measures and additives to control these odors.
Typically, the highest temperature achieved by irradiation of silicon carbide with microwave energy is relatively low, about 300° C., often because the concentration of silicon carbide must be kept low to preclude degrading the properties and characteristics of the matrix material. Higher temperatures can be achieved in combination with some materials such as solid blocks comprising silicon carbide bonded with another material. Temperatures of 600-800° C. are achieved within 3-4 minutes of exposure to microwave energy at 600 W with solid blocks of silicon nitride-bonded silicon carbide. Such material has a density lower than solid or single crystal silicon carbide. Such material is used for ashing other compounds.
There remains a need for improved radiant heating devices. Devices for food heating are limited, as are devices for other purposes. Known devices are inefficient and slow to achieve working temperatures. The slow heating rates negate the benefits, and especially the expected quick heating, of microwave and radiant heating methods. Some devices are limited by the temperatures that can be achieved. Others cause arcing when exposed to microwave energy, thus risking damage to the microwave oven or electromagnetic radiation source. The quantity of silicon carbide powder, carbon powder and particles, and other microwave-absorbing material required to achieve a desired heat generation often must be so great as to degrade the properties and characteristics of the matrix to which the material is added. Thus, there remains a need for devices that heat quickly, efficiently, and to a high temperature, without arcing and sparking, when exposed to electromagnetic radiation, especially microwave energy. Also, there exists a need for objects that have a wider range of uses than now are available.